Transistors have been widely used as important semiconductor electronic devices (electronic parts) which constitute displays and computing devices, and are currently produced by using inorganic materials such as silicon as main raw materials (semiconductor materials). In recent years, “organic transistors” (i.e., organic field effect transistors (OFET)) in which an organic material is used as the semiconductor material for the transistor have been attracting attention (for example, refer to Non-patent Document 1). Not only the OFETs are elastic and can be subjected to a low temperature treatment, and thus a flexible plastic can be used as a substrate, but also they can be produced by a printing method at a low cost, which is a great advantage. Accordingly, the OFETs have been considered as the essential electronic devices for providing indispensable items (such as flexible and low-cost terminals) in the ubiquitous era.
However, as compared to inorganic materials such as silicon, organic semiconductor materials are prone to characteristic changes and deterioration of performance when exposed to light such as visible light and ultraviolet radiation. For this reason, it is necessary to cover a semiconductor device using an organic semiconductor material (including OFETs) with a light shielding protective film. In order to take advantage of the characteristics as an organic transistor, such light shielding films need to be provided through a fine pattern formation on top of a semiconductor layer by a printing method. Moreover, it is necessary to treat the light shielding film so that the drying and curing thereof after the printing are conducted at a temperature equal to or lower than the heatproof temperature of the plastic substrate (i.e., a low-temperature curing light shielding film is required). Furthermore, the light shielding film cured at that temperature needs to exhibit a sufficient level of mechanical strength so as not to be damaged during the joining with a display panel.
Incidentally, a method for forming a light shielding film by applying a water-soluble ink containing carbon black by an ink jet process has been disclosed (for example, refer to Patent Document 1). However, it has been well known that moisture adversely affects the characteristics of OFETs (for example, refer to Non-patent Document 2). On the other hand, also in those cases where an organic solvent-based ink is used for the formation of a light shielding film, the semiconductor layer may be separated by the organic solvent. As described above, when different layers were successively printed by the conventional printing method, printing of the upper layer adversely affects the lower layer.
As a method for resolving such problems, a letterpress reverse printing method or a microcontact printing method is available (for example, refer to Non-patent Document 3). In these printing methods, not only a fine pattern can be formed, but also the lower layer is hardly affected even if an overprinting process is conducted, since an ink is applied onto an elastomeric blanket or an elastomeric stamp, and printing on a substrate is carried out after rapidly increasing the ink viscosity thereon.
However, mixing of a special ink is required in order to stably form a light shielding film for an organic semiconductor device by fine pattern printing while achieving the aforementioned various properties, that is, the light shielding property, the low-temperature curing property (curing at a temperature equal to or less than the substrate heatproof temperature), and the mechanical strength of coating film after curing by these methods.
It has been known that the following characteristics are required for the ink in order to form a fine and precise printing pattern by the letterpress reverse printing method. It is required that an ink composition have sufficient levels of viscosity and surface energy so as to be able to form a uniform ink coating film on the blanket, and exhibit sufficient levels of drying property, adhesiveness, and cohesive strength so that a complete printing pattern can be formed on top of the blanket by the time a printing pattern is formed by the contact with the letterpress plate. Moreover, the ink composition serving as a fine patterning ink composition is provided with sufficient levels of adhesiveness and cohesive strength that enable the ink coating film on the blanket to be transferred completely to a printing substrate, preferably with an ink viscosity of 5 mPa·s or less and a surface energy of 25 mN/m or less. As the ink satisfying the characteristics described above, an ink composition containing a volatile solvent, a resin that is soluble in this volatile solvent and a solid that is insoluble in this volatile solvent, and in which the volatile solvent is a mixture of a quick-drying solvent and a slow-drying solvent has been disclosed (for example, refer to Patent Documents 3 and 4). However, although Patent Documents 3 and 4 provide a detailed disclosure of an ink composition required for forming a precision pattern by the letterpress reverse printing method, there is no disclosure of a composition required for providing the aforementioned properties to the light shielding film formed with ink for forming a light shielding pattern.    Non-patent Document 1: Journal of Polymer Science: Part A: Polymer Chemistry 2002, vol. 40, pp. 3327    Non-patent Document 2: Applied Physics Letters, 2005, vol. 87, pp. 182109    Non-patent Document 3: Annual Review of Material Science, 1998, vol. 28, pp. 153    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2007-335560    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2001-56405    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2005-126608    Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2005-128346